1. Field of the Invention
The present invention relates to a color registration control based on the detection result of a patch image.
2. Description of the Related Art
Conventionally, an image forming apparatus for irradiating a plurality of photosensitive members with a laser beam to form an electrostatic latent image on each photosensitive member, developing each electrostatic latent image by toner of each color, and transferring and superimposing a plurality of toner images on a printing material or the like to form a color image has been used. In this type of image forming apparatus, due to a mechanical arrangement error of each photosensitive member, an error in light path length of a laser beam, or a change in light path length of a laser beam, a position of a printing material where each toner image is transferred may shift, thereby causing color misregistration. To deal with this, such an image forming apparatus forms a patch image for detecting color misregistration; that is, misregistration of toner images with respect to a reference color toner image, calculates an amount of the color misregistration, and corrects the color misregistration.
In a color registration control operation, a patch image is irradiated with light, and an optical sensor detects reflected light to detect the position of the patch image. More specifically, the position of the patch image is detected based on the timing when the light amount of the reflected light becomes larger or smaller than a predetermined threshold. If, therefore, the density of the patch image changes, the detected position of the patch image may become different even though the patch image is at the same position. Referring to FIG. 14, a solid line represents a change in amount of reflected light with time when a high-density patch image is irradiated with light, and a dotted line represents a change in amount of reflected light with time when a low-density patch image is irradiated with light. In FIG. 14, the difference between the densities of the patch images results in a difference Ta1 in the timing when the amount of the reflected light exceeds a threshold. The detected positions of the patch images are also different from each other.
Japanese Patent Laid-Open Nos. 10-260567 and 2010-048904 disclose a technique of stabilizing the density of a position detection patch image by forming a density control patch image before forming the position detection patch image in order to enable stable position detection.
The density is known to be high in the edge portion of a toner image in an image forming apparatus. This phenomenon in which the density is high in the edge of a toner image will be referred to as an edge density variation phenomenon hereinafter. The edge density variation phenomenon varies depending on degradation of a developer, development conditions such as a toner density, and latent image conditions such as a development contrast potential. It is, therefore, generally difficult to control an image forming apparatus so as to prevent the edge density variation phenomenon.